Top-down impact through a bottom-up mechanism. In situ effects of limpet grazing on growth, light requirements and survival of the eelgrass Zostera marina,

Description

Temporal changes in abundance, size, productivity, resource allocation and light requirements of a subtidal eelgrass (Zostera marina L.) population were followed for 2 yr after the September 1993 appearance of a previously rare oval form of the commensal limpet Tectura depicta (Berry) in Monterey Bay, California, USA. By exclusively targeting the epidermis, limpet grazing impaired photosynthetic performance but left respiratory demand, meristematic growth and more than 90% of the leaf biomass intact. The resulting low P:R ratios of grazed plants raised the light requirements for the maintenance of positive carbon balance almost 2-fold relative to healthy ungrazed plants and prevented the summertime accumulation of internal carbon reserves. Shoot density in this once-continuously vegetated 30 ha meadow declined from more than 50 shoots m-2 (2230 g fresh wt [FW] m-2) to sparse patches supporting an average of 16 shoots m-2 (380 g FW m-2). More than 50% of the continuously vegetated meadow was converted to bare sand despite ambient light availability and water temperatures that were favorable for growth of healthy, ungrazed plants. Plant size declined by 50 % and internal sugar reserves declined more than 4-fold within 6 mo after the appearance of T. depicta. Plant losses were most extensive during winter, when internal carbon reserves were minimal. The dramatic decline in eelgrass vigor and abundance reported here, despite a physical environment that was favorable for healthy eelgrass survival, illustrates the amplification of top-down control by this relatively inconspicuous limpet through a feeding mechanism that specifically impairs photosynthesis, a bottom-up process., Cited By (since 1996):11, Seaweeds, CODEN: MESED, ,

The biomass and optical properties of seagrass leaf epiphytes were measured to evaluate their potential impact on the photosynthetic performance of the seagrasses Thalassia testudinum Banks ex König (turtlegrass) and Zostera marina L. (eelgrass). Turtlegrass was obtained from oligotrophic waters near Lee Stocking Island, Bahamas; eelgrass was collected from a eutrophic environment in Monterey Bay, California. Leaf-epiphyte loads were characterized visually and quantified using measurements of their phospholipid biomass. Light absorption and reflectance of the intact epiphyte layer were determined spectrophotometrically. Turtlegrass epiphytes from the oligotrophic site absorbed a maximum of 36% of incident light in peak chlorophyll absorption bands, whereas higher epiphyte loads on eelgrass from the more eutrophic Monterey Bay absorbed 60% of incident light in peak chlorophyll absorption bands. The combination of intact epiphyte-leaf complexes and spectral measurements enabled us to construct a quantitative relationship between epiphyte biomass and light attenuation, and, by extension, between epiphyte biomass and seagrass photosynthesis. The model yielded a robust, positive relationship between epiphyte biomass and the absorption of photons in photosynthetically important wavelengths, and it generated a strong negative relationship between epiphyte biomass and spectral photosynthesis of their seagrass hosts. Furthermore, the calculations of photosynthesis highlighted the significant differences between PAR and spectral models of photosynthesis, illustrating that the spectral quality of the incident flux must be considered when evaluating the effects of epiphyte load on seagrass leaf photosynthesis. Verification of the model - using direct measurements of photosynthesis and a variety of epiphyte and macrophyte combinations from different locations - is warranted., Cited By (since 1996):34, Seaweeds, CODEN: LIOCA, ,

Cited By (since 1996):10, Ecology,
Seaweeds, CODEN: JEMBA, The effect of the common bat star, Asterina miniata (Brandt) on recruitment of the giant kelp, Macrocystis pyrifera C. Agardh, was addressed through laboratory grazing experiments, a field experiment, and modeling of feeding behavior. In the laboratory, Asterina miniata significantly decreased the density of sporophytes that developed from 1-wk-old gametophytes as well as the percent cover of 2-, 6-, and 7-wk-old sporophytes. All grazed blades remaining at the end of these experiments subsequently died. Small scale variability in spore settlement and sporophyte development were also evident in the laboratory. Bat star density significantly affected short-term kelp recruitment during a large-scale field experiment. Simple modeling suggested that high densities of Asterina miniata could graze nearly 100% of the bottom over the 90-day experiment. However, visible recruitment was seen in less than 30 days, and over this time, 36% of the substratum was predicted to have remained ungrazed. Macrocystis pyrifera of 1 to 3 cm in length may thus obtain a refuge in size from bat star grazing through rapid growth. These results indicate this generalist grazer can affect giant kelp recruitment but that even under high grazing pressure numerous plants survive. Bat star grazing probably does not contribute to large-scale differences in adult plant density but may contribute to small-scale patterns of dispersion., ,

Effects of local deforestation on the diversity and structure of southern California giant kelp forest food webs,

Description

It has been hypothesized that the high diversity of giant kelp forests is due primarily to the provision of energy and habitat by the giant kelp (Macrocystis pyrifera). In this article, I use a 19-year-long kelp forest-monitoring data set from the Channel Islands National Park (a) to identify associations between subtidal species and forested or deforested habitats, (b) to generate an idealized food web for Southern California giant kelp forests in order to identify the primary conduits of energy flow through the system, and (c) to determine changes in the diversity and complexity of this food web due to localized giant kelp deforestation. A total of 275 common species were observed in the park between 1982 and 2000, of which 36% occurred significantly more often in kelp-forested areas than in deforested areas (that is, sea urchin barrens); 25 species were found exclusively in forested areas. Most of these associations were clearly identified as trophic and/or structural associations with giant kelp itself. The producer level of the food web was diverse, although giant kelp apparently represents the greatest single source of fixed carbon through either direct grazing or the production of phytodetritus. Primary, secondary and tertiary consumer levels were also represented by numerous species, and generalist consumers were common. With deforestation, the source of primary production shifts from primarily kelps to ephemeral microalgae, macroalgae, and phytoplankton. These results support the reliance of giant kelp forest food-web structure and diversity on the presence of the forest itself., Cited By (since 1996):90, Seaweeds,
Ecology, CODEN: ECOSF, ,

The temperature dependence of NADH:NR activity was examined in several marine phytoplankton species and vascular plants. These species inhabit divergent thermal environments, including the chromophytes Skeletonema costatum (12-15°C), Skeletonema tropicum (18-25°C), Thalassiosira antarctica (-2 to 4°C), and Phaeocystis antarctica (-2 to 4°C), the green alga Dunaliella tertiolecta (14-28°C), and the vascular plants Cucurbita maxima (20-35°C) and Zea mays (20-25°C). Despite the difference in growth habitats, similar temperature response curves were observed among the chromophytic phytoplankton, with temperatures optimal for NR activity being between 10-20°C. In contrast, the chlorophyll b-containing alga and vascular plants exhibited optimal temperatures for NR activity above 30°C. Such dramatic differences in NR thermal characteristics from the two taxonomic groups reflect a divergence in NR structure that may be associated with the evolutionary diversification of chromophytes and chlorophytes. Further, it suggests a potential contribution of the thermal performance of NR to the geographic distributions, seasonal abundance patterns, and species composition of phytoplankton communities. NR partial activities, which assess the individual functions of Mo-pterin and FAD domains, were evaluated on NR purified from S. costatum to determine the possible causes for high temperature (>20°C) inactivation of NR from chromophytes. It was found that the FAD domain and electron transport among redox centers were sensitive to elevated temperatures. S. costatum cells grown at 5, 15, and 25°C exhibited an identical optimal temperature (15°C) for NADH:NR activity, whereas the maximal NR activity and NR protein levels differed and were positively correlated with growth temperature and growth rate. These findings demonstrate that thermal acclimation of NO 3 - reduction capacity is largely at the level of NR protein expression. The consequences of these features on NO 3 - utilization are discussed., Cited By (since 1996):26, Seaweeds,
Oceanography, CODEN: JPYLA, ,

Effect of temperature on photosynthesis, growth and calcification rates of the free-living coralline alga Lithophyllum margaritae

Description

Rhodolith beds are the dominant submerged calcifying aquatic vegetation in some coastal marine environments worldwide but flew quantitative data are available regarding their physiology. In the Gulf of California (Mexico), Lithophyllum margaritae (Rhodophyta, Corallinaceae) is the most abundant nongeniculate, rhodolith-forming coralline species. Over their gulf-wide distribution, rhodolidis are exposed to a wide range of seasonal temperatures (∼8-32°C). The effect of changes in temperature on the photosynthetic and calcification rates of this species is unknown. We therefore evaluated the effect of temperature (10-30°C) on the photosynthetic and calcification rates of L. margaritae rhodoliths in the lab and examined the effect of seasonal changes in temperature on growth rates in the field. Photosynthetic rates were evaluated polarographically and calcification rates were evaluated in the lab using both the buoyant weight technique and total alkalinity method, and in the field through alizarin staining. To the best of our knowledge, this is the first time that these three methods are used simultaneously to evaluate growth rates in coralline algae. Photosynthetic, calcification and growth rates showed wide fluctuations as a result of laboratory or field temperature. Photosynthetic (Pmax) and respiratory rates both increased five-fold as incubation temperature increased to 25-30°C. Similarly, calcification rates in the lab and growth rates in the field increased with higher temperatures. The lab data suggest that rhodolith growth is seasonally regulated by seawater temperature. The buoyant weight and total alkalinity techniques for determining calcification rate were comparable at low temperatures, but variability increased with temperature and this will be examined in further studies. Field growth rates, presented as apical tip extension, were significantly higher in summer (5.02 ± 1.16 mm yr-1) than in winter (0.83 ± 0.16 mm yr-1), supporting the lab results. The strong effects of temperature on photosynthetic, calcification and growth rates of Lithophyllum margaritae in the Gulf of California suggest that changes in sea surface temperature directly regulate bed production., Cited By (since 1996):13, Seaweeds, From abbreviated title field: Efecto de la temperature sobre las tasas de fotosíntesis, crecimiento y calcificación del alga coralina de vida libre Lithophyllum margaritae